1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer and the like.
2. Related Background Art
In conventional image forming apparatuses using an electrophotographic process, there are provided an image forming station at which a latent image is formed by using light, magnetism or charges and at which the latent image is visualized to obtain a visualized image, convey means for conveying a transfer material to the image forming station for permitting the image on the image forming station to be transferred onto the transfer material, and fixing means for fixing the image (transferred to the transfer material) onto the transfer material.
The image forming station includes an image forming medium in the form of an electrophotographic photosensitive body or other bodies having various features and configurations, and there are provided various means such as a latent image forming means and a developing means in association with the image forming medium.
Particularly, in many color image forming apparatuses in which a full-color image is formed by superimposing images on a transfer material by using a plurality of image forming stations, a convey means for absorbing the transfer material onto a surface of a belt and for conveying the transfer material, is used to convey the transfer material from a transfer means for transferring the images onto the transfer material to a fixing means.
A conventional example is disclosed in Applicant's Japanese Patent Laid-Open Application No. 2-13976. An example of such an image forming apparatus will be briefly explained with reference to FIG. 14.
In FIG. 14, the image forming apparatus includes three image forming stations I, II, III. Below image forming stations I, II, III, there is disposed a convey means 139 including a convey belt 126 for conveying a transfer material, and a fixing means 56 having a pair of heat rollers 56a, 56b for fixing images (on the transfer material) onto the transfer material. The fixing means 56 is disposed at an outlet of the convey means 139. The image forming stations I, II, III include photosensitive drums 111, 112, 113, chargers 114, 115, 116, developing devices 117, 118, 119, transfer chargers 120, 121, 122 and cleaners 123, 124, 125.
The convey belt 126 is formed from resin material, and a surface of the convey belt is charged by an absorb charger 133 to electrostatically absorb the transfer material during the operation so that the transfer material is stably absorbed and conveyed.
Further, while the transfer material 46 fed from a pair of register rollers 49 is being passed through the nip between a driven roller 134 and a press roller 52 (contacted with the driven roller) of the transfer material convey means 139, the transfer material is urged against the charged convey belt 126 so that the transfer material can be electrostatically absorbed onto the convey belt 126 effectively without undulation.
In this case, a conveying speed of the register rollers 49 is selected to be slightly greater than a conveying speed of the convey belt 126 so that by forming a loop in the transfer material between the pair of register rollers 49 and the press roller 52, the conveying speed of the register rollers 49 does not damage the transfer material.
Under high temperature/high humidity environmental conditions, the convey belt 126 may not be charged adequately to float the transfer material. If the transfer material is floating, deviation of transferring, deviation in registration (deviation of image forming position) or/and transfer void will occur. To avoid this, an rotatable auxiliary roller 200 and a rotatable idle roller 203 are provided in a confronting relation.
In this way, in the above-mentioned conventional example, since the auxiliary roller 200 is provided, when the images formed in the image forming stations are transferred onto the transfer material on the convey belt 126, the transfer material does not floating from the convey belt 126 and is surely absorbed onto the convey belt, with the result that a good image can be obtained without deviation of transferring, deviation in registration and transfer void.
On the other hand, if hunting or offset occurs during the movement of the convey belt 126, the deviation in image forming positions on the transfer material due to the presence of the plurality of image forming stations (referred to as "deviation in registration" herein after) is generated, and any known means for correcting hunting and offset is adopted. Among known means for correcting hunting and offset, in general, is a technique in which guide rib(s) are provided at one lateral edge or both lateral edges of the convey belt along its whole length and guide grooves are formed on some of the rollers for driving, holding and tension-applying the convey belt so that the guide ribs are guided by the guide grooves to suppress the hunting and offset.
Other than the above-mentioned conventional image forming apparatus, for example, a color image forming apparatus as shown in FIG. 15 has also widely been used. Such a color image forming apparatus will now be described briefly.
In such an electrophotographic image forming apparatus, a photosensitive drum (image bearing member) 204 is uniformly charged by a first charger (charge roller or corona charger) 214. Then, a first color electrostatic latent image is formed on the image bearing member by exposure 216, effected by an exposure device comprised of a light emitting element 213 such as a laser or an LED, in response to a first color image signal, and the latent image is visualized by a developing device 202a containing yellow (Y) developer, for example.
On the other hand, as can be understood by referring to FIG. 16, by using a drum-shaped transfer member 203 formed by covering an outer peripheral surface of a cylindrical conductive drum frame 203a by a dielectric flexible sheet 203b, bias is applied to the transfer member 203 to electrostatically absorb a supplied transfer material 201 onto the transfer member 203.
The transfer material 201 is supplied one by one by means of a sheet supply roller 214 and is pinched between the transfer member 203 and an absorb roller 215 at a predetermined timing controlled by a pair of register rollers 220. A conveying speed of the pair of register rollers 220 for conveying the transfer material 201 is selected to be slightly greater than a conveying speed of the transfer member 203 for conveying the transfer material 201. At the same time, DC voltage (as transfer bias for absorption and first color transferring) is applied to the drum frame 203a from a bias power source 217, and absorb bias is applied to the absorb roller 215 from a bias power source 218. As a result, the transfer material 201 is electrostatically held on the transfer member 203 due to charges from the absorb roller 215.
Then, the transfer material 201 is conveyed, by rotation of the transfer member 203, to a transfer position where the transfer material is opposed to the photosensitive drum 204. Consequently, the visualized image formed on the photosensitive drum 204 is transferred onto the transfer material.
Residual developer remaining on the photosensitive drum 204 is removed by a cleaner 205. Then, the photosensitive drum is uniformly charged by the first charger 214 again and an electrostatic latent image is formed on the photosensitive drum 204 by the exposure device in response to a second color image signal. The electrostatic latent image is developed and visualized by a developing device 202b containing magenta (M) developer, for example, corresponding to the second color image signal.
The second color visualized image is transferred by the bias voltage onto the same transfer material (on the transfer member 203) to which the first color visualized image was transferred. The above-mentioned process is repeated by using a third color cyan (C) developer and a fourth color black (BK) developer to form a third color visualized image and a fourth color visualized image on the photosensitive drum 204, successively, which images are in turn transferred onto the transfer material 201 in a superimposed fashion in a manner similar to the second color visualized image.
The transfer material 201 to which the four color visualized images were transferred is conveyed, by the rotation of the transfer member 203, to a separation charger 209 opposed to the peripheral surface of the transfer member 203. At this location the electrostatic absorbing force between the transfer material 201 and the flexible sheet 203b is removed by the separation charger 209, with the result that the transfer material is separated by a separation pawl 211 while electricity is being removed from the transfer material 201 by means of a separation and electricity removing charger 210. The separated transfer material 201 is directed, through a transfer material convey path, to a fixing device 206, where the images are fixed to the transfer material by the fixing device 206.
After the transfer material 201 is separated, residual developer remaining on the flexible sheet 203b, constituting the outer surface of the transfer member 203, is removed by a transfer member cleaner (not shown) and electricity on the sheet is removed by a sheet electricity removing charger 212 opposed to the sheet 203b, thereby initializing the transfer member electrically.
In this way, the visualized images are transferred onto the transfer material 201, thereby forming a color image.
However, in the conventional example shown in FIG. 14, an urging force of the press roller 52 contacted with the driven roller 134 is small. Accordingly, regarding a transfer material having normal thickness, i.e., normal stiffness (flexural rigidity), although the loop can be formed between the pair of register rollers 49 and the press roller 52 in order prevent the pair of register rollers 49 from pulling the transfer material in a direction opposite to a transfer material conveying direction during the transferring, a loop cannot be formed in a stiffer than normal transfer material.
For example, regarding a transfer material such as a thick sheet having a basis weight greater than 200 g/m.sup.2, since the stiffness of the transfer material is great, the loop is not formed between the pair of register rollers 49 and the press roller 52, with the result that the transfer material slides with respect to the convey belt 126 at the press roller 52. Consequently, the conveying speed of the transfer material is governed by the conveying speed of the pair of register rollers 49 rather than the conveying speed of the convey belt 126, and, after a tail end of the transfer material leaves the pair of register rollers 49, the conveying speed of the transfer material is governed by the conveying speed of the convey belt 126. That is to say, since the conveying speed of the transfer material passing through the transfer stations is changed, deviation in registration along a sub-scanning direction (deviation of the visualized image from a correct position to which the image is to be transferred), deviation in color or color unevenness occurs, thereby worsening image quality.
Further, the auxiliary roller 200 is also subjected to only a weak urging force sufficient to prevent the transfer material from the convey belt 126. Thus, the urging force of the auxiliary roller 200 generates only a restraining force (for the transfer material) of about 1N, but cannot generate a restraining force sufficient to form a loop in the transfer material having significant stiffness. Accordingly, the deviation in image and deviation in color due to uneven rotation cannot be prevented.
There are the following methods for forming a loop in the transfer material between the pair of register rollers 49 and the convey belt 126 or the press roller 52:
(1) The pair of register rollers 49 are disposed far away from the convey belt 126 or the press roller 52; and
(2) An enter angle of the transfer material onto the convey belt 126 given by the pair of register rollers 49 is increased.
However, in the above method (1), the pair of register rollers 49 must be disposed away from the convey belt 126 or the press roller 52 by a distance of 100 to 200 mm, with the result that the entire apparatus is made bulky.
In the above method (2), an angle of several tens of degrees must be established between the conveying direction of the transfer material given by the pair of register rollers 49 and the surface of the convey belt 126, with the result that the degree of freedom of design of an area where transfer materials supplied from a plurality of sheet supply portions such as a manual insertion tray and a sheet supply cassette are joined becomes less.
In any cases, since the apparatus becomes bulky or the layout in the apparatus is limited, the above methods are not so practical.
Further, in the conventional example shown in FIG. 14, although the guide ribs are provided on the lateral edges of the convey belt to prevent hunting and offset of the convey belt during rotation thereof, if the guide ribs themselves are provided in a hunting fashion, when the guide ribs are guided by the grooves or shoulders of the rollers for driving and tension-applying the convey belt, the hunting or staggering of the guide ribs negatively influence the rotation of the convey belt 126. That is to say, when the transfer material passes through the stations, due to the hunting or staggering of the guide ribs, the convey belt 126 is reciprocated in a direction transverse to the advancing direction of the belt, with the result that deviation in registration or deviation in color in a main scanning direction occurs, thereby worsening image quality.
On the other hand, also in the conventional example shown in FIG. 15, means for restraining the transfer material 201 with respect to the transfer member 203 does not exists in the area between the pair of register rollers 220 and the absorb roller 215. Thus, when the transfer material having normal thickness, i.e., normal stiffness (flexural rigidity), although the loop can be formed in the transfer material between the pair of register rollers 220 and the absorb roller 215, a loop cannot be formed in the transfer material having greater stiffness.
For example, regarding the transfer material having base weight greater than 200 g/m.sup.2, since the stiffness of the transfer material is great, a loop is not formed between the pair of register rollers 220 and the absorb roller 215, with the result that the transfer material slips with respect to the transfer member 203 at the absorb roller 215. Consequently, if it is expected that the transfer material having the predetermined timing given by the pair of register rollers 220 is absorbed to the predetermined position on the transfer member 203, the tip end of the transfer material advances by an amount corresponding to the slip, with the result that the image to be transferred to the transfer material deviates from the desired position. Alternatively, even after the transfer material is absorbed to the transfer member 203 in a fixed condition, before the absorption of the transfer material to the transfer member 203 is completed, due to the stiffness of the transfer material, the slip may occur at the absorb roller 215.
Accordingly, since the transfer material is absorbed on the transfer member 203 in a partially floating condition, when the image is transferred from the photosensitive drum 204, the image is not transferred onto the partially floating portion of the transfer material, with the result that image void occurs or the distorted image is transferred, thereby generating image deviation or color deviation.
There are the following methods for forming a loop in the transfer material between the pair of register rollers 220 and the absorb roller 215:
(1) The pair of register rollers 220 are disposed far away from the absorb roller 215; and
(2) An enter angle of the transfer material onto the transfer member 203 given by the pair of register rollers 220 is increased.
However, in the above method (1), the pair of register rollers 220 must be disposed far away from the absorb roller 215 by a distance of 100 to 200 mm, with the result that the entire apparatus is made bulky.
In the above method (2), when a surface passing through a nip between the transfer member 203 and the absorb roller 215 and contacting with the transfer member 203 is assumed, the conveying direction of the transfer material given by the pair of register rollers 220 must have an enter angle of several tens of degrees with respect to the contact surface, and, this cannot be established easily due to the limitation in the layout regarding the sheet supply cassette, the manual insertion portion and the transfer material convey path.
In any case, since the apparatus becomes bulky or the layout in the apparatus is limited, the above methods are not practical.